Of Mammograms and Menstrual Cycles: Timing Is Everything

Mammography may detect cancer more effectively in younger women if the test is done during the first two weeks of the menstrual cycle, according to researchers at the Fred Hutchinson Cancer Research Center in Seattle.

In a study to be published tomorrow (June 17) in the Journal of the National Cancer Institute, epidemiologist Emily White, Ph.D., and colleagues from the Hutchinson Center’s Division of Public Health Sciences, Group Health Cooperative of Puget Sound and the University of Washington are the first to report an association between menstrual phase and breast density as detected on a mammogram.

The researchers found that as the menstrual cycle progresses, breast tissue becomes less fatty (transparent) and more fibrous and dense (opaque), most likely due to fluctuations in reproductive hormones. This density, which appears as increased cloudiness on a mammogram, can make it more difficult to detect tiny, early stage malignancies among pre-menopausal women.

As a result, the researchers suggest there may be an improvement in the accuracy of mammography if women in their 40s schedule mammograms to coincide with the first two weeks of their menstrual cycle, when breast tissue is less dense and therefore more easily imaged by an X-ray. An added benefit: Breasts are less tender during this phase of the cycle, which could reduce the discomfort of mammography.

"This potentially could improve the accuracy of mammography at a relatively small cost to the woman," says White, who is also a professor of epidemiology at the University of Washington School of Public Health. She co-led the study with Stephen H. Taplin, M.D., M.P.H., an investigator in the Center for Health Studies, Group Health Cooperative of Puget Sound, and an associate professor of family medicine at UW.

To determine whether breast density varies during the menstrual cycle, White, Taplin and colleagues surveyed 2,591 women, ages 40- 49, who in 1996 underwent routine mammography screening at one of six regional mammography centers of Group Health (a Seattle-area health-maintenance organization). Women included in the study had regular menstrual periods and were not using oral contraceptives or hormone-replacement therapy.

The women’s mammograms were then read by 26 radiologists and categorized on a scale of low, medium or high density. The time within the menstrual cycle also was recorded according to follicular vs. luteal phase. The two-week follicular phase, which begins on the first day of a woman’s menstrual period, is followed by the two-week luteal phase, which is characterized by increased tissue growth and water content in the breast. Breast changes during this latter phase are thought to be caused by an increase in the hormone progesterone.

Slightly more of the women who underwent mammography during the luteal phase (28 percent for both weeks three and four) had highly dense breasts, compared with those screened during the follicular phase (24 percent for week one and 23 percent for week two).

The impact of menstrual phase on breast density was most apparent among women at or below average body weight, who naturally tend to have denser breasts. The association was not observed among women who weighed more than average for their height. "Because women with lower body mass index typically have greater breast density, they could potentially benefit most from timing their mammograms to coincide with the first two weeks of their cycle," White says.

While the Hutchinson Center study is the first to look at changes in breast density throughout the menstrual cycle, it supports recent Canadian research that indicates mammograms tend to be more sensitive and result in fewer false-negatives when scheduled during the follicular phase of a woman’s cycle.